1. Field of the Invention
This application relates generally to optical diagnostic methods and apparatus and, more specifically, to an optic neuropathy detection method and apparatus utilizing optical coherence tomography.
2. Description of Related Art
Conventional diagnostics for eye disorders typically include a detailed ophthalmic examination of the retina. For initial examination, an eye doctor will view the retina through an ophthalmoscope. For a permanent record, the retina is typically photographed with a fundus camera. A fundus photograph directly records various anatomical features of the retina, such as the optic disc, fovea, blood vessels, and lesions. The imaging capabilities of fundus photography may be enhanced by supplementary techniques. A high-contrast image of retinal blood vessels, for example, may be photographed after the injection of a fluorescent dye into the bloodstream. The resulting image is referred to as a fluorescein angiogram.
More sophisticated techniques have been developed for diagnostics of the eye. One such technique is three-dimensional optical coherence tomography (3D OCT). In this technique, a light beam is directed onto the retina. Part of the beam is back-reflected, and interferometric analysis of the back-reflected light yields information on the structure of the retina. By varying optical parameters of the light probe, features at different depths below the surface of the retina may be analyzed. With this process, an image of a cross-section of the retina may be generated by scanning the optical probe along a line on the retina. By rastering the optical probe across the surface of the retina, a series of cross-sectional images may be produced. The series of cross-sectional images may be used to characterize the 3D structure of the retina, and parameters such as local retinal thickness may be measured by 3D OCT.
Analysis of the thickness of the retina, or a portion of the retina, may be used to diagnose certain diseases of the eye, such as glaucoma. Different portions of the retina may be analyzed, including the retinal nerve fiber layer, the ganglion cell layer, the ganglion cell complex or the like. One indication of the health of the eye may be provided by comparing the retinal thickness of the patient's eye with reference data acquired from a population of healthy eyes. Progression of eye disease may also be monitored by measuring changes in retinal thickness over a period of time.
For example, a conventional approach is to utilize a circumpapillary scan to detect glaucoma and monitor a progression of glaucoma in a patient. A circle scan is performed around the optic disc at a fixed diameter. From the scan data, a thickness of the retinal nerve fiber layer around the circle is measured. However, as optic disc sizes vary from patient to patient, the fixed circle diameter needs to be sufficiently large enough to encompass the optic discs of all patients. Thus, the circle is arbitrarily sized substantially larger than the optic disc sizes of most patients. This results in useful information, which is acquired close to the optic disc, being missed. Further, when attempting to utilize circles with smaller diameters, the circle scans clip the optic disc of some patients.